Water-Based Superhydrophobic Coatings for Nonwoven and

Dec 9, 2013 - This process(21) eliminates the need for creping, and removes bulk-reducing steps. The tissue used in this article has a basis weight of...
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Water-Based Superhydrophobic Coatings for Nonwoven and Cellulosic Substrates Joseph E. Mates,† Thomas M. Schutzius,† Ilker S. Bayer,‡ Jian Qin,§ Don E. Waldroup,⊥ and Constantine M. Megaridis*,† †

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States Nanophysics, Instituto Italiano di Tecnologia (IIT), Via Morego 30, 16163 Genova, Italy § Corporate Research and Engineering, Kimberly-Clark Corporation, Neenah, Wisconsin 54956, United States ⊥ Corporate Research and Engineering, Kimberly-Clark Corporation, Roswell, Georgia 30076, United States ‡

S Supporting Information *

ABSTRACT: The intense commercial demand for efficient fluid management (e.g., water barriers) has resulted in a recent proliferation of methods intended to impart liquid repellency to various substrates. Many such methods involve wet-based processing of hydrophobic polymers and thus rely heavily on organic solvents whose properties pose environmental challenges when scaled up from the laboratory bench. The current study presents a one-step, environmentally safe (>97 wt % water), roomtemperature, low-cost, polymer-based technique that imparts superhydrophobicity to commercially relevant porous substrates. The method features aqueous dispersions of a commercially available fluoroacrylic copolymer and hydrophilic bentonite nanoclay and uses spray casting to apply coatingswhich are subsequently dried in open airto form thin conformal films. Wettability measurements demonstrate that the coating formulation imparts considerable resistance to water penetration in polymeric nonwoven and cellulosic substrates. In addition to the benign environmental impact of the aqueous formulation, all ingredients are commercially available, thus opening many technological opportunities in this area.

1. INTRODUCTION The field of superhydrophobicity has garnered a great deal of interest in recent years with numerous scientific advances propelling its remarkable growth. The intense commercial demand for efficient fluid management (e.g., water barriers)1−4 has resulted in the proliferation of methods that impart liquid repellency to various substrates. In some of these techniques, spray coating has been demonstrated as a viable application tool due to its inherent large-area low-cost capabilities and its substrate independence.5,6 Although many nonporous, untextured substrates may benefit from superhydrophobic coating treatments (e.g., hulls, exterior walls, etc.), in general, many of the envisioned applications are in the textile and nonwovens industries. Functional nanocomposite films synthesized by wet processing of polymer−particle dispersions have been widely investigated. Applications of such surface treatments include, among many others, electromagnetic interference shielding7 and liquid repellency.8−11 In superhydrophobic coating applications, the dispersion of hydrophobic particles requires harsh organic solvents, such as ethanol and acetone. Despite their functional advantages, organic solvents are environmentally undesirable in large scale industrial processes. Water-based dispersions pose little environmental concern and are therefore of higher technological interest. However, water-based approaches are severely limited in the dispersion and delivery of low-surface energy (hydrophobic) particle fillers. Due to this difficulty, the development of water-based coating formulations has been rare in the literature. Xu et al.12 reported a one-step superhydrophobic coating method for © 2013 American Chemical Society

cotton fabrics based on a modified silica hydrosol formed by cohydrolysis and co-condensation of methyl trimethoxy silane and hexadecyltrimethoxysilane (combined at ∼5 wt %). The size of the modified silica particles was adjusted by changing the ammonium hydroxide and sodium dodecyl benzenesulfonate surfactant concentrations (combined at ∼2 wt %). These researchers reported water contact angles of 151.9° and a water droplet shedding angle of 13°. In polymer−particle composite coatings, the primary function of the polymer is to serve not only as a matrix for low-surface energy filler particles10,13,14 but also as an adhesion promoter for the film on the underlying substrate. Fluoroacrylic polymer-based coatings have some very attractive properties, such as exceedingly low surface energy, low friction coefficients, repellency to both oil and water, and relatively low permeability to most gases.13 Among fluoroacrylic polymers, perfluoroalkyl methacrylate copolymers (PMCs) have been characterized for water as well as oil repellency applications.15 Due to their environmental friendliness, water-dispersed PMCs have been approved for industrial use. Although copolymers with a long perfluoroalkyl group demonstrate improved durability, they also raise environmental concerns due to adverse bioaccumulation rates16 and breakdown into PFAs (perfluorinated acids), which may pose toxicity threats. A recent study expressed concern that current maternal concentrations of perfluorooctaReceived: Revised: Accepted: Published: 222

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with one layer meltblown into a layered fabric product. The SMS used in this study has a basis weight of ∼24 g per square meter (gsm), 63% void fraction, and is made of polypropylene fiber. High-Density Paper Towel (HDPT). This is an absorbent cellulosic textile made from paper. Unlike cloth towels, paper towels are disposable and intended to be used only once. Paper towels soak up water because they are loosely woven, which enables water to wick between the fibers, even against gravity. The paper towel used herein (Kleenex hard roll towel 50606, Kimberly-Clark Corporation) has a basis weight of 38 gsm and 44% void fraction. Tissue. This tissue substrate is made by a process named the uncreped through air-dried process (UCTAD). This process21 eliminates the need for creping, and removes bulk-reducing steps. The tissue used in this article has a basis weight of around 30 gsm and 41% void fraction. Spunlace Nonwoven. Spunlacing (i.e., hydroentanglement) uses high-speed water jets to entangle fibers, thus promoting fabric integrity. Softness, drape, conformability, and relatively high strength are the major characteristics that make spunlaced products unique among all nonwovens. The spunlace material used in this study is comprised of polyethylene terephthalate (PET) fibers, has a basis weight of 50 gsm, and 58% void fraction. B. Materials. Bentonite (hydrophilic) nanoclay particles were obtained from Sigma-Aldrich. The aqueous fluoroacrylic copolymer dispersion (PMC) was obtained from DuPont (20 wt % dispersion in water; Capstone ST-100). Deionized water was used as a probe liquid for penetration pressure (hydrohead) tests, as well as the contact angle and roll-off angle measurements. C. Procedure. To prepare 100 mL of sprayable dispersion, the following steps were followed. Initially, 1.25 g of nanoclay was added to 92.5 mL of deionized water and bath-sonicated for 15 min (Branson 8200, 20 kHz, 450 w). After sonication, a stir bar was added to the dispersion and 6.25 g of the PMC aqueous solution (20 wt %) was added dropwise under mechanical mixing over the course of 1 min. The PMC was introduced slowly to ensure adequate mixing, as the solution became more viscous during this process. The final nanoclay/ PMC dispersions (97.5 wt % water, 1.25 wt % PMC, 1.25 wt % nanoclay) were applied by spray on each substrate. The 50:50 polymer-to-nanoclay solids ratio was chosen on the basis of preliminary qualitative results; increased polymer content significantly reduced surface roughness. Conversely, increased nanoclay content adversely impacted wettability as the clay is hydrophilic and requires a balanced fluoropolymer content to counteract its inherent tendency to absorb water. Likewise, increasing the overall solid content in the final dispersion increases viscosity, in turn, hindering the ability to spray; therefore, the solids wt % in the dispersion was deliberately maintained below 3% to avoid complications during spraying. An airbrush atomizer (Paasche, VL siphon feed, 0.55 mm spray nozzle) was used to spray the samples from a distance of 15 cm. Water-based spray dispersions pose several challenges as the surface roughness imparted from the particle filler, and needed for repellency, is inhibited by the slow evaporation when the substrate is wetted during spraying. To this end, the spray nozzle was chosen because it offered the finest spray atomization achievable with the employed sprayer, in turn, enhancing water evaporation during spray application. The spray distance also greatly affects coating morphology and

noic acid (PFOA) have the potential to cause adverse effects in human offspring.16 Thus, the US EPA invited fluoropolymer manufacturers to gradually eliminate precursor chemicals that can break down into PFOA. The PMC utilized in the present study was created by industry in response to this EPA initiative and will not break down into PFOAs in the environment. Although this copolymer has improved environmental properties due to its short perfluoroalkyl chain, its mechanical properties are compromised. In general, liquid repellency (as demonstrated by high droplet mobility) is characterized by low-contact angle hysteresis, which is defined as the difference between the apparent advancing (θ*A) and receding (θ*R) contact angle values.17 Superhydrophobicity (θ*A > 150°) relies on low surface energy and micro-to-nanoscale hierarchical surface roughness. Most methods used to designate superhydrophobicity are based on nondynamic or quasi-dynamic (ratio of inertia to surface tension forces ∼ 10−5) contact angle measurements, which is hardly appropriate for designating liquid repellency under realistic situations. However, high θ*A values alone are not a reliable measure of liquid repellency (see SLIPS18 surfaces, rose petal effect,19 etc.). In many applications, the coated substrate is expected to withstand more than just a thin film of fluid or a few distributed droplets, therefore a more robust approach and testing method is required to gauge a coating’s liquid resistance. Liquid penetration resistance is a good metric of measuring repellency and depends upon substrate morphology and porosity. This paper demonstrates a water-based superhydrophobic coating for use on, but not limited to, commercially relevant polymeric nonwoven and cellulosic porous substrates. The specific substrates were selected as being representative of a plethora of products in the market, ranging from consumer and industrial tissues and paper towels, to diapers and medical protective fabrics. The coating is a composite of a fluoroacrylic copolymer and hydrophilic bentonite nanoclay that is applied by spray. The use of the hydrophilic filler is counterintuitive but it facilitates the water-based formulation pursued in this work. An apparatus has been constructed to challenge the substrates with liquid pressures that are slowly increased until reaching the upper limit of resistance to water penetration (i.e., hydrohead). The water-repelling effectiveness of the dry coating is measured in terms of contact angles and hydrohead and is compared for a series of substrates. The study utilizes these water-based polymeric dispersions to reduce substrate effective pore size and solid wettability, with the ultimate goal of raising resistance to externally applied water pressure. In coupling the maximum water penetration data with the metrics of superhydrophobicity, the study sheds light on the connection between liquid repellency and liquid penetration resistance20 for surfaces modified with successive surface treatments. The results demonstrate that it is possible to achieve a stable, waterbased dispersion that achieves superhydrophobicity when applied and dried on commercially relevant nonwoven and cellulosic substrates. It should be noted that the present composite coatings are not superoleophobic, although the presence of the fluoropolymer may impart some degree of oil repellency.

2. EXPERIMENTAL SECTION A. Substrates. Spunbond/Meltblown/Spunbond (SMS) Nonwoven. SMS nonwovens combine two layers of spunbond 223

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ments were repeated at least five times (different sample cutouts) for a given substrate and coating level. Extra care was taken to rule out the possibility of any time-dependence of water breakthrough at rising column pressures, as would have been the case if column pressure was maintained at a given level and the water had time to work its way into, and through, the sample. Thus, if a given substrate was introduced to column pressures below the breakthrough pressure, it could withstand those pressures for a prolonged period. Water column pressure was increased in increments of a few centimeter head at a time and was allowed to remain there for up to 5 min at a time before increasing it further and repeating this process until the limit was reached. In the determinations of contact angles (CA), the irregularities in surface topography of the substrates made it difficult to obtain accurate and consistent measurements. This is due to the sample having surface features that were not much smaller than the test droplet size, as well as some of the substrates incorporating fibers that extended through the surface treatment, thus obscuring the contact line between droplet and substrate. Sample measurements and images are shown in Figure S1 (Supporting Information) to show the uneven nature of these coated surfaces and demonstrate that even with the substrate texture, rough CA values visibly exceeded 150° (the acceptable threshold for superhydrophobicity). To obtain these measurements, 3 × 1 cm2 coated sample strips were cut and attached to a glass slide using double-stick tape. To gauge the effectiveness of the coating at different add-on levels, smooth glass slides were sprayed in the exact same manner as the textured test substrates. The latter approach allowed for accurate measurement of the CA (see Figure 1) for a given coating add-on mass without being impeded by surface topography generated in the manufacturing processes of the individual rough substrates.

deposition weight and, therefore, was chosen to avoid excessive substrate wetting during spraying, as would be the case if the spray distance were too small, and maximize coating deposition rate, which is reduced at greater spray distances. Care was taken to ensure spray uniformity and control over coating thickness. To determine coating weight and verify uniformity, glass slides were placed within the typical spray area normally occupied by the substrates during spraying. These slides were subsequently weighed on a per spray-pass basis to evaluate uniformity and determine amount of coating deposited in gsm. Each spray pass represented an average of ∼0.6 gsm of coating applied onto glass slides; this base level was used as a gauge for determining approximate coating deposition onto the nonuniform porous substrates. D. Characterization. Each of the samples, coated or uncoated, were characterized for hydrohead resistance and droplet contact/sliding angles. The difficulties in measuring contact angle hysteresis on substrates with roughness features of the same length scale as the test droplet sizes is addressed below as well as in the Supporting Information. In addition, Scanning Electron Microscopy (SEM) images were acquired to analyze coating levels, coating uniformity, and effective substrate pore size. It should be noted that these soft substrates (all disposable) are not intended for long-term or repeated use, thus attention was paid only to their single-use performance. Nonetheless, the applied coatings did not shed during normal handling. The applied coating levels were quantified so as to ensure consistent coating properties over the entire sample area (9.5 × 7 cm2). Four spray passes resulted in a uniformly distributed coating mass of 2.4 ± 0.2 gsm over that sample area. At coating add-on levels below 2.4 gsm, individual droplets beaded upon contact but were absorbed into the underlying hydrophilic substrates after a few seconds. At or above 2.4 gsm, the water droplets remained beaded consistently for a prolonged period without absorption. The 2.4 gsm coating was then tested for all four substrates as the lowest coating level. Two other coating levels were also tested; 4.8 ± 0.2 gsm and 9.6 ± 0.2 gsm. These three coating levels represented light, medium, and heavy addons and were within the requirements of industrial applications (preferably below 10 gsm). At the light and medium add-on levels, the coatings were nearly imperceptible and did not change the as-received apparent feel and softness of the substrates. At the heaviest coating level, there is stiffening of the substrates from the increased polymer presence as well as perceptible roughening of the surface from the increased presence of clay particles. For the hydrohead tests, a simple water column device was constructed; see the Supporting Information. The apparatus consisted of a 2.5 cm diameter graduated column placed over the 2.5 cm circular sample secured coating-side-up at the bottom of the column. A rubber gasket placed on top of the sample ensured that any liquid penetration would have to occur through the surface coating and then the porous sample itself. The column was slowly filled with water until the first breakthrough (water passed through sample) was visible, at which point, the height of the water column defined the maximum hydrohead (in centimeters) endured by the sample. For the two hydrophilic samples (tissue, HDPT), water resistance for the uncoated case was naturally zero; water penetrated and absorbed into the sample immediately. The two inherently hydrophobic samples (SMS, spunlace) had nonzero hydrohead values before any coating was applied. Measure-

Figure 1. Water droplet dispensed from a flat-tipped needle (top) on a glass slide coated with 4.8 gsm of PMC/nanoclay composite. The needle diameter is 1 mm.

For sliding angles, the substrate strips affixed to glass slides were placed on a goniometer operated by a small DC servomotor, which allowed precise control as it stepped through small angle increments until the water droplets were seen to roll down the sample. If the droplets did not roll, the coated substrates were deemed “sticky” and were marked with “S” in Table 1. The volume of droplets used in these tests was ∼10 μL. The dimples and other surface irregularities on a given 224

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Table 1. Sliding Angle Measurements for Three Add-on Coating Levels for Each of the Four Substratesa 2.4 ± 0.2 gsm 4.8 ± 0.2 gsm 9.6 ± 0.2 gsm

SMS

HDPT

spunlace

tissue

S S S

28.3° ± 12.1° 26.6° ± 5.6° 30.5° ± 10.6°

27.8° ± 4.3° S S

30.5° ± 8.7° 23.7° ± 4.7° 25.4° ± 6.0°

a

If droplets were not observed to roll, the substrates were designated as ‘sticky’ (marked with an S).

substrate again interfered with the natural roll-off during these measurements, and caused the significant standard deviations listed in Table 1. For SEM observations, a small area (roughly 0.25 cm2) was cut from each treated and untreated sample for comparison. The samples were then sputter-coated with a 4 nm Pt−Pd coating to facilitate SEM imaging.

Figure 3. Substrate landscape: SEM images of four uncoated substrates at the same magnification. The scale bar applies for all images. (a) SMS, hydrophobic substrate. The spunbond polypropylene fibers can be clearly discerned from the meltblown fibers beneath, which have much smaller diameters. (b) HDPT, hydrophilic substrate. The density of these fibers presents an advantage in water resistance after coating. (c) Tissue, hydrophilic substrate. Although very similar to HDPT, these fibers are slightly smaller and less dense but weaker in terms of mechanical strength. (d) Spunlace, hydrophobic substrate made of PET fibers.

3. RESULTS AND DISCUSSION The four types of substrates featured morphologies that varied according to their manufacture and intended use. SEM was employed to characterize samples before and after coating (Figures 2 and 3). The cellulosic fibers had rough and irregular

from Δp = 2σ cos θ*A (1/Dmin + 1/Dmax), where σ denotes the surface tension of water (72 mN/m at 20 °C). Because the exact values of the contact angle cannot be determined on each substrate, but are well within the superhydrophobic regime (>150°), we approximated |cos θ*A| ≈ 1. The experimental determination of the Laplace pressure was made from the measured hydrohead (i.e., minimum water column height at which water penetrated the substrate; see the Supporting Information). The two values for each substrate are compared in Table 2, which shows that the theoretical estimate exceeds Table 2. Laplace Pressure Calculations for All Four Substrates, as Based on Pore Size Ranges Attained from SEM Image Analysis for a 2.4 gsm Coatinga substrate

Dmin range (μm)

Dmax range (μm)

theoretical Laplace pressure (kPa)

SMS HDPT spunlace tissue

5−10 10−25 15−25 25−40

20−25 40−80 40−65 70−85

20−36 8−18 8−13 5−8

experimental hydrohead (kPa) 3.4 1.7 1.3 0.5

± ± ± ±

0.4 0.1 0.2 0.2

a

These are compared with experimental values derived from hydrohead measurements.

Figure 2. (a) SEM image displaying porosity of HDPT coated with 4.8 gsm PMC/nanoclay. (b) Depiction of how effective pore size (orthogonal maximum, minimum diameters Dmax and Dmin) was determined to calculate the theoretical Laplace pressure needed to push the liquid through the pore.

the experimental value by an order of magnitude. This discrepancy is attributed to the underestimation of the maximum pore dimension from the SEM images; large pores critically influence the hydrohead, and due to their isolated presence could be difficult to detect by SEM when analyzing sample areas significantly smaller (∼0.25 mm2) than the surface area challenged during hydrohead pressure measurements (5.07 cm2). Nonetheless, the data in Table 2 clearly indicate a correlation between pore size and penetration pressure; as the former grows, the latter is reduced. This supports the hypothesis that pore morphology is the primary factor in designating water barrier resistance. The effective pore size of

surfaces (see Figures 2 and 3b,c). In contrast, the polymeric fibers had regular cylindrical geometries with smooth surface features (see Figure 3a,d). Using the SEM images, coating uniformity was verified and the effective pore size was estimated and averaged at several locations across each substrate. The estimates of the minimum and maximum pore diameters (Dmin, Dmax) were then employed to calculate an approximate theoretical Laplace pressure needed to penetrate the pore 225

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induced curvature of the softer substrate could cause fibers to stretch and pores to widen. The added coating actually strengthened the robustness of the fibers, thus not permitting deformation of the intrinsic pores. Overall, the addition of increasing coating levels for a given substrate imparts optimal hydrohead performance before the coating begins to inhibit the properties of the material (e.g., breathability). After a certain point, the addition of more coating material becomes counterproductive; the goal of applying such coatings is to create the best performance with the least expenditure of material and resources. Although the measurement of contact and sliding angles are critical to assessing superhydrophobicity, imparting barrier resistance by spraying an aqueous dispersion was a focal point of this study. A secondary goal was to examine the relationship between increasing hydrophobicity and water barrier resistance. The data suggests that superhydrophobicity is entirely dependent upon surface energy and roughness, and can be achieved with a minimal coating add-on. On the other hand, water resistance is entirely dependent on substrate porosity and can be enhanced only through coating add-on insofar as the pore sizes are reduced (see Figure S2, Supporting Information). After analyzing the sliding angles for each substrate in Table 1, it became evident that some of the substrates displayed poor sliding angle performance while possessing extremely high CAs and, more importantly, excellent resistance to water pressures. SMS maintained the highest water resistance from the outset, yet it was observed to be “sticky” by measure of droplet roll-off angles. Figure 5 shows the nonuniform coating of SMS compared to that of HDPT, a hydrophilic substrate. The waterbased coating had difficulty spreading across the hydrophobic SMS fibers, thus leaving whole or partial fibers exposed during the curing process (Figure 5a). This causes water droplets to stick when moving over or resting across these features of varying surface energies, and does not allow for a consistent coating of the fibrous pore after successive coatings. This also accounts for the poor water droplet mobility, and the relatively minimal increase (when compared with the other three substrates) in hydrohead performance from light to heavy coatings (Figure 4). Although select materials may possess some aspects of both super-repellency and water resistance, these properties are mutually exclusive and one does not necessarily guarantee the other. The effective pore size of each substrate, determined by fiber spacing and diameter, is ultimately the limiting factor in designating water resistance.

Figure 4. Experimentally measured water penetration resistance (hydrohead) for each substrate at three coating levels: 2.4 ± 0.2, 4.8 ± 0.2, and 9.6 ± 0.2 gsm. The first two substrates, SMS and spunlace, are inherently hydrophobic and have a nonzero water resistance asreceived (uncoated). The other two substrates are hydrophilic, and thus have zero resistance in their uncoated state.

each substrate was reduced by the coating addition, thus causing an increase in hydrohead resistance. Figure 4 presents the hydrohead data for each of the four substrates in their uncoated state as well as with light (2.4 gsm), medium (4.8 gsm), and heavy (9.6 gsm) coatings. The addition of the superhydrophobic coating consistently increased water resistance. However, when CA is compared against coating thicknesses (Table 3), the trend is not clear. Even for the light Table 3. Sessile Contact Angle Measurements on Glass Slides Coated with the Same Add-on Levels as the Cellulosic and Nonwoven Substratesa coating weight CA on glass slide

2.4 ± 0.2 gsm 158.8° ± 4.5°

4.8 ± 0.2 gsm 166.2° ± 1.4°

9.6 ± 0.2 gsm 155.3° ± 7.5°

a

All coating levels lie within the superhydrophobic range with minimal deviance.

coating (∼2.4 gsm), very high contact angles were observed, and these increased only slightly for the medium coating, while remaining approximately the same for the heavy coating. The first two substrates (SMS and spunlace) are inherently hydrophobic, with precoat water resistance values given by the black bar in Figure 4. The relatively slight increase in water resistance for each coating level on SMS is regulated by the smaller pore sizes of the middle, meltblown, layer. As shown in Figure 3a, the meltblown fibers and their spacing are much smaller with respect to the two outer, spunlaid layers and constitute the primary factor affecting the substrate’s inherent water resistance (∼32 cm). The coatings were applied only on the outer layer and reduction in that larger pore size due to coating did not impact, other than superficially, the inner layer’s effective pore size. The tissue and HDPT substrates were wettable as-received and their initial water resistance was thus zero (Figure 4). An initial jump in water resistance was seen after a light coating (∼2.4 gsm) on all substrates, and subsequent less pronounced increases were seen for spunlace and HDPT as the coating levels were doubled. The jump in water resistance for tissue from the low to the middle coating level (2.4 to 4.8 gsm) may be due to an initial mechanical weakness of this substrate. As pressure was increased, the

4. CONCLUSION This study reported a water-repellent coating formulation based on an aqueous spray composed of 97.5% water and commercially available chemicals, namely, a fluoroacrylic copolymer and hydrophilic bentonite nanoclay. The wet processing relied on spray application and drying, both performed at room temperature. The dried coating imparts superhydrophobicity (contact angles over 150°) at minimum add-on levels of 2.4 gsm. The results indicate the lack of a correlation between water droplet beading (repellency) and water barrier resistance (hydrohead). A naturally hydrophobic substrate showed high water barrier resistance despite having poor water mobility after coating application. It was shown that resistance to water penetration pressure is derived primarily from substrate porosity (fiber spacing and diameter). Although the coating formulation increases water resistance, especially in the case of inherently hydrophilic substrates, this increased 226

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(2) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Effects of the surface roughness on sliding angles of water droplets on superhydrophobic surfaces. Langmuir 2000, 16, 5754. (3) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Progess in superhydrophobic surface development. Soft Matter 2008, 4, 224. (4) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. Superhydrophobic surfaces: from structural control to functional application. J. Mater. Chem. 2008, 18, 621. (5) Ogihara, H.; Okagaki, J.; Saji, T. Facile Fabrication of Colored Superhydrophobic Coatings by Spraying a Pigment Nanoparticle Suspension. Langmuir 2011, 27, 9069. (6) Wu, W. C.; Wang, X. L.; Liu, X. J.; Zhou, F. Spray-Coated Fluorine-Free Superhydrophobic Coatings with Easy Repairability and Applicability. ACS Appl. Mater. Interfaces 2009, 1, 1656. (7) Das, A.; Hayvaci, H. T.; Tiwari, M. K.; Bayer, I. S.; Erricolo, D.; Megaridis, C. M. Superhydrophobic and conductive carbon nanofiber/ PTFE composite coatings for EMI shielding. J. Colloid Interface Sci. 2011, 353, 311. (8) Manoudis, P. N.; Karapanagiotis, I.; Tsakalof, A.; Zuburtikudis, I.; Panayiotou, C. Superhydrophobic composite films produced on various substrates. Langmuir 2008, 24, 11225. (9) Bayer, I. S.; Tiwari, M. K.; Megaridis, C. M. Biocompatible poly(vinylidene fluoride)/cyanoacrylate composite coatings with tunable hydrophobicity and bonding strength. Appl. Phys. Lett. 2008, 93, 173902. (10) Tiwari, M. K.; Bayer, I. S.; Jursich, G. M.; Schutzius, T. M.; Megaridis, C. M. Highly Liquid-Repellent, Large-Area, Nanostructured Poly(vinylidene fluoride)/Poly(ethyl 2-cyanoacrylate) Composite Coatings: Particle Filler Effects. ACS Appl. Mater. Interfaces 2010, 2, 1114. (11) Das, A.; Schutzius, T. M.; Bayer, I. S.; Megaridis, C. M. Superoleophobic and conductive carbon nanofiber/fluoropolymer composite films. Carbon 2012, 50, 1346. (12) Xu, L.; Zhuang, W.; Xu, B.; Cai, Z. Superhydrophobic cotton fabrics prepared by one-step water-based sol-gel coating. J. Text. Inst. 2012, 103, 311. (13) Tiwari, M. K.; Bayer, I. S.; Jursich, G. M.; Schutzius, T. M.; Megaridis, C. M. Poly(vinylidene fluoride) and Poly(ethyl 2cyanoacrylate) Blends through Controlled Polymerization of Ethyl 2-Cyanoacrylates. Macromol. Mater. Eng. 2009, 294, 775. (14) Schutzius, T. M.; Bayer, I. S.; Tiwari, M. K.; Megaridis, C. M. Novel Fluoropolymer Blends for the Fabrication of Sprayable Multifunctional Superhydrophobic Nanostructured Composites. Ind. Eng. Chem. Res. 2011, 50, 11117. (15) Ma, M. L.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Superhydrophobic fabrics produced by electrospinning and chemical vapor deposition. Macromolecules 2005, 38, 9742. (16) Martin, J. W.; Whittle, D. M.; Muir, D. C. G.; Mabury, S. A. Perfluoroalkyl contaminants in a food web from lake Ontario. Environ. Sci. Technol. 2004, 38, 5379. (17) Xu, X. M.; Wang, X. P. The modified Cassie’s equation and contact angle hysteresis. Colloid Polym. Sci. 2013, 291, 299. (18) Wong, T. S.; Kang, S. H.; Tang, S. K. Y.; Smythe, E. J.; Hatton, B. D.; Grinthal, A.; Aizenberg, J. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 2011, 477, 443. (19) Feng, L.; Zhang, Y. A.; Xi, J. M.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal effect: A superhydrophobic state with high adhesive force. Langmuir 2008, 24, 4114. (20) Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Robust omniphobic surfaces. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 18200. (21) Wendt, G. A.; Chiu, K. F.; Burazin, M. A.; Farrington, T. E.; Heaton, D. A. Method of Making Soft Tissue Product. United States Patent 5672248, September 30, 1997.

Figure 5. SEM images of SMS and HDPT substrates with 2.4 gsm (light) coating. (a) SMS fiber from the top, spunbond, layer displaying nonuniform coating; the raw fiber can be seen underneath the clay− polymer coating. (b) HDPT fibers are completely and uniformly covered with the light coating.

resistance is bounded by the Laplace pressure associated with the effective pore size.

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ASSOCIATED CONTENT

S Supporting Information *

Typical images captured during CA measurements for all four samples at three coating add-on levels; images showing pore size as a function of add-on coating level for two representative substrates: hydrophilic HDPT and hydrophobic SMS,; and schematic of hydrohead apparatus consisting of pump reservoir, viewing mirror and column. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*C. M. Megaridis. Email: [email protected]. Tel: +1 312 996-3436. Fax: +1 312 413 0447. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by Kimberly-Clark Corp., which also contributed the test substrates. REFERENCES

(1) Marmur, A. The lotus effect: Superhydrophobicity and metastability. Langmuir 2004, 20, 3517. 227

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